1. Field of the Invention
The present invention relates to a method and apparatus for producing an electrophotographic photoreceptor designed for use in an image forming apparatus such as a copying machine and a printer.
2. Description of the Related Art
An electrophotographic photoreceptor designed for use in an image forming apparatus such as a copying machine, a printer and a facsimile machine, is commonly fabricated by coating the outer peripheral surface of a hollow cylindrical substrate with organic photosensitive layers. In consequence of repeated developments directed toward providing higher-performance electrophotographic photoreceptors to meet the recent demand, most of the developed electrophotographic photoreceptors have come to comprise a photosensitive layer having a laminated structure formed by comprising or coating an intermediate layer, a charge generating layer, and a charge transporting layer in order. Further, though only in some electrophotographic photoreceptors, a protective layer for enhancing durability is added on the outermost layer of the laminated structure.
In the present specification, a layer composed of the intermediate layer, the charge generating layer, the charge transporting layer, and the protective layer will be collectively referred to as “photosensitive layer”. Note that the intermediate layer and the protective layer are provided solely to achieve performance improvement, and are therefore not essential for the electrophotographic photoreceptor. Thus, a layer composed of the charge generating layer and the charge transporting layer, as well as a layer of the charge generating layer and the charge transporting layer that are integrated as a single layer, will also be referred to as a photosensitive layer. In some cases, part of the laminated structure including the charge generating layer and the charge transporting layer, as well as part of the layer including the charge generating layer and the charge transporting layer that are integrated as a single layer, will be exclusively referred to as “photoconductive layer”.
The photosensitive layer to be provided in the electrophotographic photoreceptor is required to take the form of a thin film having a uniform thickness. In order to ensure high performance in the electrophotographic photoreceptor by applying a light coating of the photosensitive layer in uniform thickness, and also to achieve coating at lower cost, research and development have been under way to come up with innovative coating techniques.
As techniques for coating with the photosensitive layer the outer peripheral surface of a cylindrical substrate acting as an electrophotographic photoreceptor elementary tube, there have hitherto been known several coating methods such as spray coating method, dip coating method, and blade coating method. However, such conventional coating techniques pose problems, for example coating unevenness and poor productivity.
For example, according to the spray coating method, a coating operation is performed through the use of a spray nozzle for ejecting a coating solution in the form of a microscopic particle. In this case, while the resultant coating is satisfactory from outer appearance, in reality a coating layer obtained through one-time coating has a quite small thickness. Therefore, repeated coating operations are required to achieve a desired layer thickness. Moreover, when a large quantity of coating solution is applied at one time, the coating solution falls in drops, which results in formation of a coating layer having an uneven thickness. Furthermore, during the coating operation with a jet of coating solution, the volatile constituent present in the coating solution is easily volatilized. This causes an undesirable increase in the viscosity of the coating solution, in consequence whereof there results an orange-peel surface on the resultant coating layer (i.e. a phenomenon in which undulations such as found in an orange peel appear on the surface of the layer).
On the other hand, according to the dip coating method, a cylindrical substrate acting as an electrophotographic photoreceptor elementary tube is immersed in a coating solution, with its one end kept retained in position, while the axis of the cylinder is maintained in a vertical alignment with respect to the fluid level. Upon raising the sunken cylinder after a certain period of time, the surface of the cylindrical substrate is coated with a photosensitive layer. This method has been widely used for production of electrophotographic photoreceptors. However, in the case of adopting the dip coating method, the layer thickness (hereafter also referred to as “film thickness”) of a coating is highly dependent on, for example the rate at which the cylindrical substrate is pulled out from the coating solution; the viscosity of the coating solution; and the rate at which the volatile constituent contained in the coating solution evaporates. These factors need to be strictly controlled accordingly. Moreover, since the cylindrical substrate is raised from the coating solution in a vertical direction, it follows that the coating solution drips down from the surface of the cylindrical substrate due to the influence of gravity. As a result, the applied coating film becomes thinner at the upper end of the cylindrical substrate relative to the lower end thereof, when viewed in the raising direction. Furthermore, a coating solution portion remaining at the lower end of the cylindrical substrate tends to dry out inadequately. Therefore, if the subsequent coating is applied before the existing coating layer dries out sufficiently, the two coating solutions will be mixed with each other, which may cause a so-called contamination.
In order to eliminate such unevenness of the film thickness, the raising rate needs to be controlled strictly. However, besides the difficulty in controlling the raising rate, there is a fundamental problem that a coating film having a uniform thickness cannot be obtained without lowering the rate at which the cylindrical substrate is raised after dipping has been performed. Another problem is that the coating is needlessly applied also to the interior and the end face of the cylindrical substrate. There will thus emerge a need for removing the unnecessary coating films formed in the aforementioned regions. In addition, since the cylindrical substrate is wholly immersed in the coating solution, the coating solution needs to be stored in a reservoir at all times in an amount large enough for the cylindrical substrate to be soaked throughout its length. That is, there is a need to constantly prepare the coating solution in a larger amount than is necessary for coating-film formation. This leads to an undesirable decrease in coating solution utilization efficiency. In an attempt to increase the coating solution utilization efficiency, instead of preparing a new coating solution every time a coating operation is performed, reuse of the existing coating solution has been brought into practice. In this case, a newly prepared coating solution is added only in a required amount in the used coating solution stored in the reservoir every time a coating operation is performed. However, the viscosity and characteristics of the coating solution vary with time, and also vary with the addition of the newly prepared coating solution because of a subtle difference between the two fluid materials. As a result, there will be a necessity to optimize the coating conditions every time a coating operation is performed, which leads to poor operation efficiency.
According to the blade coating method, a coating operation is performed as follows. At the outset, a blade is arranged in the proximity of the cylindrical substrate so as to face the cylindrical substrate. Then, the blade is supplied with a coating solution so as to apply the coating solution to the cylindrical substrate. After one turn of the cylindrical substrate, the blade is driven to retract. While this method yields high productivity, during the retraction of the blade, part of the coating film applied to the cylindrical substrate is upheaved due to the surface tension of the coating solution. This leads to unevenness in film thickness.
Aside from the coating methods described just above, there have been known a roll coating method and an ink-jet coating method. According to the roll coating method, onto a coating roll is formed a coating solution film whose thickness is controlled properly. Then, the cylindrical substrate is arranged in the proximity of or in abutment with the coating roll so as to face the coating roll. By rotating the cylindrical substrate and the coating roll respectively, it is possible to allow the coating roll to print-coat the coating solution onto the cylindrical substrate. On the other hand, according to the ink-jet coating method, a nozzle is arranged so as to face the cylindrical substrate. A coating operation is performed with a jet of ink droplets ejected from the nozzle.
The roll coating method is advantageous in its coating solution utilization efficiency, that is, it requires only a small quantity of base coating solution for production. However, the roll coating method has also drawbacks. When the coating roll and the cylindrical substrate are separated from each other after the coating operation, an excess of the coating solution is prone to adhere to the cylindrical substrate due to the surface tension of the coating solution, which is a so-called fluid-trailing phenomenon. Moreover, the resultant coating film is seamed due to the fluid-trailing phenomenon, in consequence whereof there results unevenness in film thickness. This leads to occurrence of image defects. Note that the seam that appears on the coating film refers to surface unevenness (uneven film thickness) resulting from adhesion of the excessive coating solution, which takes place in accompaniment with the separating motion between the coating roll and the cylindrical substrate.
In order to prevent occurrence of the seam, several related art techniques have been proposed to date. For example, according to Japanese Unexamined Patent Publication JP-A 3-12261 (1991), upon completion of one or more turns of the cylindrical substrate to bring the coating operation to an end, the cylindrical substrate is moved away from a coating supply roll, while the cylindrical substrate is driven to rotate continuously so as to bring about a leveling effect on the coating surface (to make the film thickness uniform). In the method disclosed in Japanese Unexamined Patent Publication JP-A 3-12261, however, there is a need to allow for the amount of a clot of coating solution to be leveled off so as to achieve highly accurate film-thickness control. Another problem is that the seam cannot be removed readily to perfection.
For example, according to Japanese Unexamined Patent Publication JP-A 11-216405 (1999), the seam is prevented from occurring by reducing the film thickness of a coating material put on the coating roll in accompaniment with the separating motion between the coating roll and the cylindrical substrate. Moreover, according to Japanese Unexamined Patent Publication JP-A 2000-325863 (2000), after a coating operation, the relationship between the film thickness of a coating material put on the coating roll and a gap left between the coating roll and the cylindrical substrate is defined. In conformity with the relationship, the amount of the coating material put on the coating roll is reduced, so that the coating material portion laid across the coating roll and the cylindrical substrate can be cut off. However, neither of the methods disclosed in Japanese Unexamined Patent Publication JP-A 11-216405 and Japanese Unexamined Patent Publication JP-A 2000-325863 succeeded in lessening the seam to the extent of preventing occurrence of image defects with perfection. Furthermore, in the case of adopting such methods, it is necessary to control the coating conditions strictly during the coating and separating operations. This makes it impossible to provide high productivity.
Moreover, for example, Japanese Unexamined Patent Publication JP-A 11-276958 (1999) proposes exercising control of the pace at which the coating roll and the cylindrical substrate are spaced apart after a coating operation. However, the technique disclosed in Japanese Unexamined Patent Publication JP-A 11-276958, alike to those disclosed in Japanese Unexamined Patent Publications JP-A 11-216405 and JP-A 2000-325863 described above, failed to lessen the seam to the extent of preventing occurrence of image defects with perfection.
Further, according to Japanese Unexamined Patent Publication JP-A 2000-84472 (2000), a rib is created on the coating film by causing a difference in circumferential velocity between the coating roll and the cylindrical substrate. In this state, the coating roll and the cylindrical substrate are separated from each other. However, the method disclosed in Japanese Unexamined Patent Publication JP-A 2000-84472 has drawbacks, too. In creating a rib on the coating film, if a solvent having a low boiling point is used, it will be impossible to secure a sufficient leveling time required to remove the rib so that the film thickness can be made uniform, in consequence whereof there results undulations on the coating film. By way of contrast, if a solvent having a high boiling point is used, while the leveling time can be secured adequately, much time needs to be spent in a drying operation. As a result, the productivity is significantly deteriorated. Furthermore, formation of a desired rib cannot be achieved without determining various coating conditions strictly including a roll diameter; circumferential velocity; a gap; the viscosity of a coating material; and surface tension. Besides the difficulty in determining such coating conditions, there is a problem that the composition of a coating solution and apparatus configuration are limited in their range of adjustment. In particular, considering the small film thicknesses of the charge generating layer and the intermediate layer, it could be extremely difficult to determine conditions to be fulfilled for forming a desired rib. Granted that a rib is formed satisfactorily, since it will dry out in a short period of time, it follows that a sufficient leveling time cannot be secured. This makes it difficult to obtain a layer having a uniform thickness.
On the other hand, the ink-jet method for coating is through the use of a minute nozzle. A jet of coating solution droplets ejected from the nozzle is applied to a target object under coating. There have been known a few types of nozzle head structures, namely, a few ways of ejecting a jet of coating solution, as exemplified by the piezoelectric method and the bubble jet (registered trademark) method/thermal method. According to the former, a coating solution is forced to jet out under the vibration pressure of a piezo element. According to the latter, electric power is applied to a heater to cause a temperature rise, thereby creating bubbles in a coating solution. The coating solution is forced to jet out under the expansion pressure of the bubbles. The ink-jet coating method possesses several advantages. For example, a jet of coating solution droplets is allowed to fly rectilinearly with high accuracy. Moreover, since the ejecting actions of a plurality of nozzles can be controlled individually, it follows that no masking process is required; wherefore the coating efficiency can be increased significantly. Further, replacement of the coating solution can be made simply by changing an ink storage tank, and the coating solution can be utilized to the fullest. This makes it possible to provide extremely high productivity.
As a related art practice for forming a coating film on the surface of a columnar or cylindrical target object under coating by the ink-jet coating method, Japanese Unexamined Patent Publication JP-A 2-272567 (1990) proposes one that performs a coating operation by driving a to-be-coated object to rotate in a horizontally-retained state while moving a discharge nozzle along the surface of the to-be-coated object in a direction of the rotation axis of the to-be-coated object. The example disclosed in Japanese Unexamined Patent Publication JP-A 2-272567 employs tetrahydrofuran alone as a solvent for a coating solution. However, since the boiling point of tetrahydrofuran is unduly low and the solvent is therefore volatilized soon, its use poses a risk that the coating solution dries out at the discharge portion of the discharge nozzle, whereby clogging is induced in the nozzle. In addition to that, the aimed leveling effect is not adequately exerted on the coating.
As another related art practice related to the ink-jet coating, Japanese Unexamined Patent Publication JP-A 11-19554 (1999) proposes one that performs coating on a to-be-coated object with use of coating solution droplets which are forced to fly continuously in streak form from a plurality of minute nozzles under pressure. In the method disclosed in Japanese Unexamined Patent Publication JP-A 11-19554, however, the nozzles cannot be controlled individually. Furthermore, since a pump for applying pressure and the discharge nozzle are connected to each other by a tube, there arises a time lag between pressurization and discharge. Thus, in contrast to the other ink-jet coating method that allows individual nozzle control, this method is disadvantageous in accuracy and response.
Here, the to-be-coated object is assumed to be an electrophotographic photoreceptor. In a case where an approximately 20 to 40 μm-thick charge transporting layer is formed by the ink-jet coating method, since coating solution droplets are ejected from the nozzle which is as small as a few tens of micrometers (μm), a plurality of coating layers need to be formed by repeated coating operations to attain the desired thickness. This slows down the coating process, thereby deteriorating the productivity significantly. Moreover, because of the difficulty in applying the coating solution in uniform thickness, the nozzle tends to suffer from clogging due to drying. This makes it difficult to allow the coating solution to be discharged with stability for a longer period of time.
Meanwhile, although the ink-jet coating method is suitable for forming a charge generating layer having a small thickness, considering the fact that a layer to be obtained is a thin film, it will be difficult to control the physical properties of the coating solution used as a material to form the thin film. In general, in a high-viscosity coating solution, while uniformity of liquid composition can be ensured readily because of its resistance to pigment precipitation, the dischargeability is poor. By way of contrast, in a low-viscosity coating solution, while a satisfactory dischargeability can be ensured, pigments are prone to precipitation and coagulation that will eventually cause nozzle clogging.
Moreover, in general, a charge generating layer and a charge transporting layer are separately formed by coating in different coating devices that are spaced apart. To achieve this, not only a conveyance system for conveying a to-be-coated object to each of the coating devices, but also a carrier for transferring the to-be-coated object to the conveyance system need to be prepared for use. This gives rise to a problem of an undesirable increase in equipment investment cost.
Hence, in the production of electrophotographic photoreceptors, highly efficient production method and apparatus have been sought after that enable both a charge generating layer and a charge transporting layer to be formed in a single, common device and require less coating solution than ever.